Assume that I bought 100 warrants @ $0.50 for $50

...

I could buy 100 shares @ $0.90 for $90

You only have to lay out $50 with the warrants to catch the same upside as you got with $90 in common stock.

Now, in your scenario you do pay the exercise price. But (1) it only has to be paid if the warrant doesn't expire worthless, (2) it has to be paid in the future and most importantly (3) you don't actually have to pay it. Somebody will gladly buy it at the end of that year for a tiny (fraction of a percentage) discount to the stock price minus the strike. That way they can make a very quick and virtually riskless return (which adds up if you do it all the time) and you don't actually have to lay put any liquidity to exercise.